An intrusion detection system using a radio-wave radiation unit arranged in a cable form such as a leaky coaxial cable (hereinafter referred to as a “leaky cable”) is based on the following principles.
For example, when a leaky cable is placed so as to enclose the perimeter of premises, the surroundings of the premises become a monitoring area. And, by generating an electric field around the leaky cable, when an intrusion object enters the premises thereacross, a domain of the electric field formed by the leaky cable is disturbed crosswise, so that a fluctuation of the electric field occurs owing to the intrusion object. By capturing the electric-field fluctuation, and also by detecting a position in which the electric-field fluctuation occurs, it is possible to find out the intrusion position.
Detection of an intrusion object according to the electric-field fluctuation is performed, for example, as follows.
With one end-point of a leaky cable for transmission being made as a feed end, a signal having pulse waves is inputted from the feed end and radiated from the leaky cable as a radio wave. While keeping constant space-intervals with the leaky cable for transmission, a leaky cable for reception is placed so that it receives a radio wave having been radiated from the leaky cable for transmission. As for the leaky cable for reception, a receiver is connected thereto at an end-point thereof, which is on the same side as the feed end of the leaky cable for transmission. By this receiver, a radio-wave signal received via the leaky cable for reception is received.
A signal through the medium of a radio wave that is radiated at a position near the feed end of the leaky cable and received thereat arrives fast at the receiver; a signal passing through a position away from the feed end and closer to the termination end arrives late at the receiver. Namely, a pulse wave that is a reception signal inputted into the receiver exhibits a waveform expanded in terms of time in comparison to the one having been transmitted.
While observing an envelope of the reception signal expanded in terms of time, when intrusion occurs, part of the envelope corresponding to the intrusion location exhibits an amplitude fluctuation. The presence of the intrusion object is detected from the amplitude fluctuation, and an intrusion position is traced from a location in which the envelope changes (for example, refer to Patent Document 1).
[Patent Document 1] Japanese Laid-Open Patent Publication No. H10-95338